Progress with the Garching (West Germany) 30 meter prototype for a gravitational wave detector

Mechanical noise, thermally or seismically driven, in a 30 m gravitational wave detector was considerably reduced by separate (extremely simple) suspensions for the two near mirrors and the beam splitter. Fluctuations in laser beam geometry were efficiently reduced by using a single-mode fiber that ends in the vacuum on a (separately suspended) beam-steering block. Laser frequency stabilization was considerably improved in a second control loop, using the total optical path in the interferometer as reference. The interferometer was operated with 50 and 110 beams in the delay line (optical paths of 1.5 and 3.3 km, respectively). From 500 Hz to 6kHz, the noise measured is close to the shot noise for the 100 mW of available light power. Resonance peaks around 10 kHz were identified as being due to the Pockels cell mounting. From 1 kHz downward, a gradual increase in noise is observed, the origin of which is not known. Improvements in the isolation from ground noise and increases in light power will help to identify the sources of the remaining excess noise.